Electrode for Secondary Battery

ABSTRACT

An electrode for a secondary battery includes a current collector, a first electrode mixture layer disposed on at least one surface of the current collector and including styrene butadiene rubber, and a second electrode mixture layer disposed on the first electrode mixture layer and including a second styrene butadiene rubber. The first styrene butadiene rubber and the second styrene butadiene rubber have a repeating unit of styrene derived structure and a repeating unit of a butadiene derived structure, the first styrene butadiene rubber containing 40 to 90 mol % of a butadiene monomer based on total content of a monomer, and the second styrene butadiene rubber having a lower content of a butadiene monomer than the content of the first styrene butadiene rubber.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.17/375,055, filed on Jul. 14, 2021, which claims priority to KoreanPatent Application No. 10-2020-0087785 filed Jul. 15, 2020, thedisclosures of which are hereby incorporated by reference in theirentireties.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to an electrode for a secondary battery,and more particularly, to an electrode for a secondary battery havingimproved adhesion.

Description of Related Art

As technological development and demand for mobile devices increase,demand for secondary batteries as an energy source is rapidlyincreasing, and among such secondary batteries, lithium secondarybatteries having a high energy density and voltage are commerciallyavailable. Such a secondary battery has a structure in which anelectrode assembly capable of being charged and discharged while havinga positive electrode/separator/negative electrode structure is mountedon a battery case, and in this case, the electrodes of positive andnegative electrodes are manufactured by applying an electrode activematerial or the like to one or both surfaces of a metal currentcollector, followed by being dried and rolled.

Currently, an electrode plate having high adhesive strength may bemanufactured with polyvinylidene fluoride (PVdF) widely used as a binderfor positive and negative electrodes. However, PVdF covers the activematerial in the same state as the polymer fibers are full, and thus,deteriorates the battery performance inherent in the electrode activematerial in terms of capacity and efficiency. In addition, in the caseof PVdF that lacks flexibility, when a material having a large specificsurface area and a high expansion and contraction rate during chargingand discharging, like natural graphite or a metal-based active material,is used as an electrode active material, the bonding tends to be brokenand the cycle characteristics tend to be deteriorated.

In addition, when the amount of active material loading per unit area ofthe electrode increases, the active material layer is pushed duringrolling and the loading amount per unit area is lower than theoriginally intended value. Accordingly, there is a problem in whichmanufacturing costs increase as the overall thickness of the electrodeis further increased to obtain the required capacity. Accordingly, thereis a high need for developing a negative electrode that may improve theperformance of a secondary battery and decrease the resistance withinthe electrode due to an increase in adhesion between the negativeelectrode current collector and the active material and the cohesionbetween the active material and the active material.

SUMMARY OF THE INVENTION

This summary is provided to introduce a selection of concepts insimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

An aspect of the present disclosure is to provide an electrode for asecondary battery, capable of lowering electrical resistance of asecondary battery while securing adhesion between an electrode currentcollector and an active material and cohesion between the activematerial and the active material, and a secondary battery including thesame.

According to an aspect of the present disclosure, an electrode for asecondary battery includes a current collector, a first electrodemixture layer disposed on at least one surface of the current collectorand including carboxymethyl cellulose and styrene butadiene rubber, anda second electrode mixture layer disposed on the first electrode mixturelayer and including carboxymethyl cellulose. A weight average molecularweight of the carboxymethyl cellulose included in the first electrodemixture layer is lower than a weight average molecular weight of thecarboxymethyl cellulose included in the second electrode mixture layer.

The weight average molecular weight of the carboxymethyl celluloseincluded in the first electrode mixture layer may be 40×10⁴ to 300×10⁴.

The weight average molecular weight of the carboxymethyl celluloseincluded in the second electrode mixture layer may be 350×10⁴ to600×10⁴.

The styrene butadiene rubber included in the first electrode mixturelayer may include 40 to 90 mol % of a butadiene monomer based on a totalcontent of a monomer. The second electrode mixture layer may furtherinclude styrene butadiene rubber.

A weight of the styrene butadiene rubber included in the secondelectrode mixture layer may be equal to or less than a weight of styrenebutadiene rubber included in the first electrode mixture layer.

The styrene butadiene rubber included in the second electrode mixturelayer may include 5 to 35 mol % of a butadiene monomer based on a totalcontent of a monomer.

The first electrode mixture layer may include 0.6 to 2.0 weight % ofcarboxymethyl cellulose and 2.0 to 5.0 weight % of styrene butadienerubber based on a total weight of the first electrode mixture layer.

The second electrode mixture layer may include 0.6 to 2.0 weight % ofcarboxymethyl cellulose and 2.0 weight % or less of styrene butadienerubber based on a total weight of the second electrode mixture layer.

A thickness of the first electrode mixture layer may be 10 to 40 μm.

A thickness of the second electrode mixture layer may be 10 to 100 μm.

The electrode may be a negative electrode.

BRIEF DESCRIPTION OF DRAWING

The above and other aspects, features, and advantages of the presentinventive concept will be more clearly understood from the followingdetailed description, taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 schematically illustrates an electrode according to an exemplaryembodiment of the present disclosure.

DESCRIPTION OF THE INVENTION

The following detailed description is provided to assist the reader ingaining a comprehensive understanding of the methods, apparatuses,and/or systems described herein. However, various changes,modifications, and equivalents of the methods, apparatuses, and/orsystems described herein will be apparent to one of ordinary skill inthe art. The sequences of operations described herein are merelyexamples, and are not limited to those set forth herein, but may bechanged as will be apparent to one of ordinary skill in the art, withthe exception of operations necessarily occurring in a certain order.Also, descriptions of functions and constructions that would be wellknown to one of ordinary skill in the art may be omitted for increasedclarity and conciseness.

The features described herein may be embodied in different forms, andare not to be construed as being limited to the examples describedherein. Rather, the examples described herein have been provided so thatthis disclosure will be thorough and complete, and will fully convey thescope of the disclosure to one of ordinary skill in the art.

Herein, it is noted that use of the term “may” with respect to anembodiment or example, e.g., as to what an embodiment or example mayinclude or implement, means that at least one embodiment or exampleexists in which such a feature is included or implemented while allexamples and examples are not limited thereto.

Throughout the specification, when an element, such as a layer, region,or substrate, is described as being “on,” “connected to,” or “coupledto” another element, it may be directly “on,” “connected to,” or“coupled to” the other element, or there may be one or more otherelements intervening therebetween. In contrast, when an element isdescribed as being “directly on,” “directly connected to,” or “directlycoupled to” another element, there can be no other elements interveningtherebetween.

As used herein, the term “and/or” includes any one and any combinationof any two or more of the associated listed items.

Although terms such as “first,” “second,” and “third” may be used hereinto describe various members, components, regions, layers, or sections,these members, components, regions, layers, or sections are not to belimited by these terms. Rather, these terms are only used to distinguishone member, component, region, layer, or section from another member,component, region, layer, or section. Thus, a first member, component,region, layer, or section referred to in examples described herein mayalso be referred to as a second member, component, region, layer, orsection without departing from the teachings of the examples.

Spatially relative terms such as “above,” “upper,” “below,” and “lower”may be used herein for ease of description to describe one element'srelationship to another element as illustrated in the figures. Suchspatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, an element described as being “above” or “upper”relative to another element will then be “below” or “lower” relative tothe other element. Thus, the term “above” encompasses both the above andbelow orientations depending on the spatial orientation of the device.The device may also be oriented in other manners (for example, rotated90 degrees or at other orientations), and the spatially relative termsused herein are to be interpreted accordingly.

The terminology used herein is for describing various examples only, andis not to be used to limit the disclosure. The articles “a,” “an,” and“the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. The terms “comprises,” “includes,”and “has” specify the presence of stated features, numbers, operations,members, elements, and/or combinations thereof, but do not preclude thepresence or addition of one or more other features, numbers, operations,members, elements, and/or combinations thereof.

Due to manufacturing techniques and/or tolerances, variations of theshapes illustrated in the drawings may occur. Thus, the examplesdescribed herein are not limited to the specific shapes illustrated inthe drawings, but include changes in shape that occur duringmanufacturing.

The features of the examples described herein may be combined in variousmanners as will be apparent after gaining an understanding of thedisclosure of this application. Further, although the examples describedherein have a variety of configurations, other configurations arepossible as will be apparent after gaining an understanding of thedisclosure of this application.

The drawings may not be to scale, and the relative sizes, proportions,and depiction of elements in the drawings may be exaggerated forclarity, illustration, and convenience.

Hereinafter, exemplary embodiments of the present disclosure will bedescribed with reference to various examples. However, embodiments ofthe present disclosure may be modified into various other forms, and thescope of the present disclosure is not limited to the embodimentsdescribed below.

According to an exemplary embodiment, an electrode for a secondarybattery is provided, and in detail, an electrode for a secondary batteryhaving improved adhesion is provided.

A binder used in manufacturing the electrode is required to haveadhesion properties in which adhesion between the current collector andthe active material and cohesion between the active materials may besimultaneously provided, and is a factor that may affect the capacityreduction and stability of the secondary battery if adhesion is notsecured. To secure such adhesion properties, it may be considered toincrease the content of the binder or employ a method of using a binderhaving relatively stronger adhesion. However, in this case, inevitably,there is a disadvantage of increasing electrical resistance of thesecondary battery.

Accordingly, the present inventors manufactured an electrode having amultilayer structure by using a binder having upper and lower layershaving different properties, to secure adhesion between the electrodecurrent collector and the active material as well as cohesion betweenthe active materials and simultaneously reduced resistance within theelectrode, thereby realizing the present disclosure.

According to an exemplary embodiment of the present disclosure, anelectrode for a secondary battery includes a current collector 10; afirst electrode mixture layer 20 disposed on at least one surface of thecurrent collector 10 and including carboxymethyl cellulose and styrenebutadiene rubber; and a second electrode mixture layer 30 disposed onthe first electrode mixture layer 20 and including carboxymethylcellulose, and a weight average molecular weight of carboxymethylcellulose contained in the first electrode mixture layer 20 is lowerthan a weight average molecular weight of carboxymethyl cellulosecontained in the second electrode mixture layer 30.

In an exemplary embodiment of the present disclosure, carboxymethylcellulose is used as a first binder and styrene butadiene rubber is usedas a second binder to secure both adhesion between the electrode currentcollector and the active material and the cohesion between the activematerial layers.

The first binder, carboxymethyl cellulose, has a difference in theproperties thereof depending on the difference in the weight averagemolecular weight. According to an exemplary embodiment of the presentdisclosure, the weight average molecular weight of carboxymethylcellulose included in the first electrode mixture layer may preferablybe lower than the weight average molecular weight of carboxymethylcellulose contained in the second electrode mixture layer.

In detail, the carboxymethyl cellulose of a high molecular weight mayimprove the adhesion between the electrode current collector and theactive material, but has relatively high electrical resistance andrelatively low dispersibility, whereas the carboxymethyl cellulose of alow molecular weight may have slightly lower adhesive strength than thatof the high molecular weight carboxymethyl cellulose, but has relativelylow electrical resistance and excellent dispersibility. Accordingly, byusing a weight average molecular weight of the carboxymethyl cellulosecontained in the first electrode mixture layer 20, lower than the weightaverage molecular weight of the carboxymethyl cellulose contained in thesecond electrode mixture layer 30, the adhesion between the electrodecurrent collector and the active material may be improved and electricalresistance may also be improved.

The weight average molecular weight of carboxymethyl cellulose includedin the first electrode mixture layer 20 may be 40×10⁴ to 300×10⁴, indetail, 60×10⁴ to 200×10⁴. If the weight average molecular weight ofcarboxymethyl cellulose included in the first electrode mixture layer 20is lower than 40×10⁴, the viscosity of the slurry may be relatively lowand the coating process may be difficult, and if exceeding 300×10⁴,electrical resistance may be relatively high and undissolved substancesmay be included, and thus, it may be difficult to obtain the effect ofthe present disclosure for improving the cohesion between activematerials and reducing the resistance within the electrode.

The weight average molecular weight of carboxymethyl cellulose includedin the second electrode mixture layer 30 may be 350×10⁴ to 600×10⁴, indetail, 350×10⁴ to 450×10⁴. If the weight average molecular weight ofcarboxymethyl cellulose included in the second electrode mixture layer30 is less than 350×10⁴, the cohesion between the active material layersis low, and partial detachment of the active material layer may occur ina notching process, and the lifespan characteristics of the battery mayalso be deteriorated. If exceeding 600×10⁴, electrical resistance may besignificantly increased due to an undissolved product of carboxymethylcellulose, and thus battery characteristics may be deteriorated.

On the other hand, the first electrode mixture layer 20 may containstyrene butadiene rubber as a second binder, and the second electrodemixture layer 30 may also contain styrene butadiene rubber as a secondbinder if necessary. In this case, it may be preferable that theproperties of the styrene butadiene rubber included in the first andsecond electrode mixture layers may also be different from each other.According to an embodiment, the amount of styrene butadiene rubberincluded in the second electrode mixture layer may be equal to or lessthan the amount of styrene butadiene rubber included in the firstelectrode mixture layer.

The styrene butadiene rubber used as the second binder in the presentdisclosure refers to a polymer including a repeating unit of astyrene-derived structure and a repeating unit of a butadiene-derivedstructure. In the styrene butadiene rubber, the repeating unit of thebutadiene-derived structure may be a repeating unit of, for example, astructure derived from 1,3-butadiene or a derivative thereof, forexample, 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, or2-ethyl-1,3-butadiene, or the like. In addition, the repeating unit ofthe styrene-derived structure may be a repeating unit of a structurederived from styrene which is an aromatic vinyl compound, such asstyrene, α-methylstyrene, p-methyl styrene, 3-methylstyrene,4-methylstyrene, 4-propylstyrene, 1-vinylnaphthalene,4-cyclohexylstyrene, 4-(p-methylphenyl) styrene,1-vinyl-5-hexylnaphthalene, or the like, or a structure of a derivativethereof.

In general, in styrene butadiene rubber, styrene and butadiene maintainmechanical strength and exhibit adhesive properties. The styrenebutadiene rubber used in the secondary battery according to anembodiment of the present disclosure may be obtained preferably bysynthesizing by adding a monomer capable of imparting friendlyproperties with an electrolyte in addition to styrene and butadiene toimprove battery characteristics, and the monomer is not particularlylimited, and for example, an acrylate-based monomer, anacrylonitrile-based monomer, and the like may be used.

On the other hand, in an embodiment of the present disclosure, thestyrene butadiene rubber of the first electrode mixture layer 20 maycontain 40 to 90 mol % of butadiene monomer based on the total contentof the monomer, and in more detail, may contain 50 to 85 mol % ofbutadiene monomer. If the content of butadiene monomer is less than 40mol %, interlayer adhesion between the current collector and the activematerial may be relatively low, and thus, peeling of an outermostpunched surface may occur at the time of immersion thereof in theelectrolyte. If the content thereof exceeds 90 mol %, electricalresistance is high and the affinity with the electrolyte is low,resulting in the occurrence of a problem in which battery resistanceincreases.

On the other hand, the styrene butadiene rubber of the second electrodemixture layer 30 may contain 5 to 35 mol % of butadiene monomer, indetail, 5 to 30 mol %, based on the total content of the monomer. If thecontent of butadiene monomer is less than 5 mol %, the mechanicalstrength of the styrene butadiene rubber is lowered, and thus, adhesionmay decrease, and the lifespan characteristics of the battery may bedeteriorated. If the content thereof exceeds 35 mol %, since theaffinity with the electrolyte is low, it may be difficult to obtain theeffect of the present disclosure for reducing the resistance within theelectrode.

In this manner, by using styrene butadiene rubber having a relativelyhigh content of styrene butadiene monomer as a second binder in thefirst electrode mixture layer 20 that is in contact with the electrodecurrent collector 10, the adhesion to the electrode current collectormay be improved. By using styrene butadiene rubber having a low contentof styrene butadiene monomer for the second electrode mixture layer 30,electrical resistance may be significantly reduced overall.

On the other hand, in the first electrode mixture layer 20, based on thetotal weight of the first electrode mixture layer 20, carboxymethylcellulose may be included in an amount of 0.6 to 2.0% by weight, indetail, 0.8 to 1.8% by weight. If the content of carboxymethyl celluloseincluded in the first electrode mixture layer 20 is less than 0.6% byweight, it may be difficult to disperse the active material, andaccordingly, the viscosity is low, and the processability of the coatingprocess may be deteriorated, whereas the content exceeds 2.0% by weight,the viscosity is too high, and the flowability is low, and accordingly,the processability may be deteriorated, and battery characteristics mayalso be deteriorated.

In addition, in the first electrode mixture layer 20, based on the totalweight of the first electrode mixture layer 20, the styrene butadienerubber may be included in an amount of 2.0 to 5.0% by weight, in detail,in an amount of 2.0 to 3.5% by weight. If the content of the styrenebutadiene rubber contained in the first electrode mixture layer 20 isless than 2.0% by weight, detachment may occur in the notching processdue to low adhesion, and if the content exceeds 5.0% by weight,electrical resistance may increase and battery characteristics may bedeteriorated.

In the second electrode mixture layer 30, based on the total weight ofthe second electrode mixture layer 30, carboxymethyl cellulose may beincluded in an amount of 0.6 to 2.0% by weight, in detail, 0.8 to 1.8%by weight. If the content of carboxymethyl cellulose included in thesecond electrode mixture layer 30 is less than 0.6% by weight, it maydifficult to secure the cohesion between the active material layers, andscrap and partial detachment may occur in the notching process, whereasif the content exceeds 2.0 weight %, it may be difficult to obtain theeffect of the present disclosure for improving the cohesion between theactive materials and reducing the resistance within the electrode due tohigh electrical resistance.

In addition, in the second electrode mixture layer 30, based on thetotal weight of the second electrode mixture layer 30, styrene butadienerubber may be included in an amount of 2.0% by weight or less, indetail, in an amount of 1.5% by weight or less. If the content of thestyrene butadiene rubber contained in the second electrode mixture layer30 exceeds 2.0% by weight, electrical resistance is increased, andbattery characteristics may be deteriorated.

On the other hand, the thickness of the first electrode mixture layer 20may preferably be 10 to 40 μm. If the thickness is less than 10 μm,scratching of the active material or tearing of the current collectormay occur in the coating process, and if the thickness exceeds 40 μm,the effect of the present disclosure may be reduced due to the use of anexcessive amount of SBR having a high styrene butadiene content. Inaddition, the thickness of the second electrode mixture layer 30 maypreferably be 10 to 120 μm. If the thickness is less than 10 μm, sincethe thickness is similar to the primary particle size of the electrodeactive material, it may be difficult to perform the coating process. Onthe other hand, if the thickness exceeds 120 μm, the concentrationgradient of the styrene butadiene rubber between the first electrodemixture layer and the second electrode mixture layer is relativelygreat, the effect of the present disclosure may not appear. Accordingly,in an exemplary embodiment of the present disclosure, the thicknessratio of the first electrode mixture layer and the second electrodemixture layer may be 1:1 to 3. On the other hand, the mixture density ofthe first electrode mixture layer and the second electrode mixture layerhaving the above thickness may be 1.65 g/cc.

A method of manufacturing an electrode for a secondary battery accordingto an exemplary embodiment of the present disclosure is not particularlylimited, and may be performed by a known method. For example, afterforming a first electrode mixture layer by applying and drying a firstslurry including an electrode active material, a binder and a conductivematerial in a solvent onto the electrode current collector by a methodsuch as bar coating, casting, spraying or the like; a second slurryincluding an electrode active material, a binder and a conductivematerial in a solvent is applied to the first electrode mixture layerand dried, by a method such as bar coating, casting, spraying or thelike, thereby manufacturing an electrode for a secondary battery.

As the solvent, for example, dimethyl sulfoxide (DMSO), isopropylalcohol, N-methylpyrrolidone (NMP), acetone, or water may be used. Inthe case of the amount of the solvent used, in consideration of thecoating thickness of the composition for the formation of an electrodeactive material layer and a manufacturing yield, the solvent amount maybe sufficient as long as it may dissolve and disperse the electrodeactive material, the conductive material and the binder and it mayprovide a viscosity capable of exhibiting excellent thickness uniformityat the time of being subsequently applied to form the first electrodeactive material layer.

On the other hand, the electrode may be a negative electrode, and thesecondary battery including the negative electrode according to thepresent disclosure has reduced resistance in the electrode, and as aresult, the capacity and lifespan characteristics of the battery may besignificantly improved.

As the negative electrode active material, at least one carbon-basedmaterial selected from, for example, crystalline artificial graphite,crystalline natural graphite, amorphous hard carbon, low crystallinesoft carbon, carbon black, acetylene black, Ketjen black, super P,grapheme and fibrous carbon, an Si-based material, a metal complex oxidesuch as LixFe₂O₃ (0≤x≤1), Li_(x)WO₂ (0≤x≤1), Sn_(x)Me_(1-x)Me′_(y)O_(z)(Me: Mn, Fe, Pb, Ge; Me′: Al, B, P, Si, elements of groups 1, 2 and 3 ofthe periodic table, halogen; 0<x≤1; 1≤y≤3; 1≤z≤8) or the like, a lithiummetal, a lithium alloy, a silicon alloy, a tin alloy, metal oxides suchas SiO, SiO₂, SnO, SnO₂, PbO, PbO₂, Pb₂O₃, Pb₃O₄, Sb₂O₃, Sb₂O₄, Sb₂O₅,GeO, GeO₂, Bi₂O₃, Bi₂O₄, and Bi₂O₅, a conductive polymer such aspolyacetylene or the like, a Li—Co—Ni based material, a titanium oxide,or the like may be used.

The conductive material is used to impart conductivity to the electrode,and is not particularly limited as long as it has conductivity withoutcausing side reactions with other elements of the secondary battery.Detailed examples of the conductive material may include graphite suchas natural graphite, artificial graphite or the like; carbon-basedmaterials such as carbon black, acetylene black, ketjen black, channelblack, furnace black, lamp black, thermal black, carbon fiber or thelike; metal powder particles or metal fibers such as copper, nickel,aluminum, silver or the like; conductive whisker such as zinc oxide andpotassium titanate or the like; conductive metal oxides such as titaniumoxide; or a conductive polymer such as a polyphenylene derivative or thelike, and one thereof alone or a mixture of two or more may be used.

Example

Hereinafter, the present disclosure will be described in more detailwith reference to examples. The following examples are for explainingthe present disclosure in more detail, and the present disclosure is notlimited thereby.

1. Preparation of Negative Electrode

A first negative electrode slurry in which artificial graphite is usedas a negative electrode active material was coated and dried on a copperfoil to form a first negative electrode mixture layer, and a secondnegative electrode slurry was coated and dried on the first negativeelectrode mixture layer to prepare a negative electrode. Carboxymethylcellulose was used as the first binder, and styrene butadiene rubber wasused as the second binder. The weight ratios of the first and secondbinders of the first electrode mixture layer and the second electrodemixture layer were 1.2:1.5 equally. The contents and molecular weightsof carboxymethyl cellulose and styrene butadiene rubber contained in thefirst negative electrode mixture layer and the second negative electrodemixture layer were controlled and are illustrated in Table 1.

The molecular weight of carboxymethyl cellulose illustrated in Table 1is 0.05 wt %, which is a value analyzed by gel permeation chromatography(GPC) through a dissolution and filtering process.

2. Manufacturing of Lithium Secondary Battery

A positive electrode was manufactured by coating and drying a slurrycontaining LiCoO₂ as a positive electrode active material on an aluminumfoil. After interposing a polyolefin separator between the positiveelectrode and the negative electrode prepared above, an electrolytesolution in which 1 M of LiPF₆ was dissolved was injected into a solventin which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixedin a volume ratio of 30:70, thereby manufacturing a coin-type lithiumsecondary battery.

3. Adhesion Measurement Experiment

(1) Measurement of Adhesion Between Negative Electrode Current Collectorand Negative Electrode Mixture Layer

After cutting the negative electrode prepared above into a size of 18 mmin width and 150 mm in length, a tape having a width of 18 mm wasattached to the negative electrode current collector, and wassufficiently adhered thereto using a roller having a load of 2 kg.Thereafter, the negative electrode mixture layer was adhered to one sideof a tensile tester (DS2-50N by IMADA) using a double-sided tape, andthen the tape attached to the negative electrode current collector wasfastened to the opposite side of the tensile tester to measure theadhesion. The results are illustrated in Table 1.

(2) Measurement of Cohesion Between Negative Electrode Mixture Layers

The surface of the negative electrode mixture layer prepared above wascut by 10 μm or more using sandpaper of 1000 grit or more, and then cutinto the size of 18 mm in width and 150 mm in length. Next, a tapehaving a width of 18 mm was attached to the negative electrode mixturelayer, and sufficiently adhered using a roller having a load of 2 kg.Thereafter, the negative electrode mixture layer was adhered to one sideof the tensile tester using a double-sided tape, and then the tapeattached to the negative electrode current collector was fastened to theopposite side of the tensile tester to measure the adhesion thereof. Theresults are illustrated in Table 1.

(3) Measurement of Cycle Capacity Retention Rate

After the manufactured coin-type lithium secondary battery had a restingtime for 10 hours, charging and discharging were performed once at 0.1C-rate to perform a formation process. Thereafter, 0.1 C-rate chargingand discharging were performed twice more to measure the cycle capacityof the manufactured battery, and whether or not there was a defect wasconfirmed. The cycle capacity retention rate was measured by repeating0.3 C-rate charging and discharging. The results are illustrated inTable 1.

TABLE 1 Adhesion 2nd negative 1st negative 2nd negative 1st negativebetween electrode electrode electrode mixture electrode mixture negativemixture layer mixture layer layer (Upper layer) layer (lower layer)electrode Cohesion Cycle (upper layer) (lower layer) CMC molecular CMCmolecular current between active capacity CMC/SBR CMC/SBRweight/Butadiene weight/Butadiene collector and material and retentionComposition Composition content of SBR content of SBR active materialactive material rate (% (wt %) (wt %) (mol %) (mol %) (N/18 mm) (N/18mm) @100 cycle) Comparative 1.2/0.5 1.2/2.5  3 million/80 mol %  3million/80 mol % 0.30 ± 0.05 1.40 ± 0.10 89.8 Example 1 Comparative1.2/0.5 1.2/2.5 3.5 million/80 mol % 3.5 million/80 mol % 0.30 ± 0.051.45 ± 0.10 90.2 Example 2 Comparative 1.2/0.5 1.2/2.5  3 million/80 mol%   350,000 /80 mol % 0.10 ± 0.05 1.30 ± 0.10 85.2 Example 3 Comparative1.2/0.5 1.2/2.5  3 million/80 mol % 4.2 million/80 mol % 0.30 ± 0.081.40 ± 0.10 88.9 Example 4 Example 1 1.2/0.5 1.2/2.5 4.2 million/35 mol%  3 million/80 mol % 0.30 ± 0.05 1.50 ± 0.10 95.0 Example 2 1.2/0.51.2/2.5 4.2 million/20 mol %  3 million/80 mol % 0.30 ± 0.05 1.50 ± 0.1097.1 Example 3 1.2/0.5 1.2/2.5 4.2 million/20 mol %  3 million/40 mol %0.20 ± 0.05 1.50 ± 0.10 95.3 Example 4 1.2/0.5 1.2/2.5 4.2 million/20mol % 1.4 million/80 mol % 0.35 ± 0.03 1.50 ± 0.10 99.2 Example 51.2/0.0 1.2/3.0 4.2 million/—     1.4 million/80 mol % 0.35 ± 0.03 1.40± 0.10 99.7

In Example 1, it can be confirmed that by applying CMC having amolecular weight of 4.2 million to the upper layer and CMC having amolecular weight of 3 million to the lower layer, not only the adhesionof the electrode was significantly improved, but also the capacityretention rate was significantly improved compared to that of thecomparative example. From this, it can be seen that the molecular weightof CMC has a great influence on the adhesion of the electrode.

As can be seen in Examples 2 and 3 in which the butadiene content of SBRof the upper layer is 20 mol %, it can be confirmed that the cyclecapacity retention rate is improved as the mol % of butadiene in the SBRof the second negative electrode mixture layer decreases. In detail, itcan be seen that in the case of the butadiene content of the SBR in thelower layer, Example 2 was also superior to Example 3 in which 40 mol %was used, in terms of adhesion characteristics and lifespancharacteristics.

Referring to Example 4, it can be seen that CMC having a molecularweight of 1.4 million and SBR having a butadiene content of 80 mol %were used in the lower layer, and thus, the electrode adhesion wasimproved, and CMC having a molecular weight of 4.2 million and SBRhaving a butadiene content of 20 mol % were used in the upper layer, andthus, the resistance characteristics were improved and the lifespancharacteristics were also improved.

Example 5 was provided by controlling the CMC/SBR contents of the upperand lower layers, and it could be confirmed that as compared withExample 4, as the content of SBR of the lower layer was increased, theadhesion was further improved, and as SBR, which may act as resistance,was not added to the upper layer, the lifespan characteristics wereimproved.

In Comparative Example 1 in which the molecular weight of CMC containedin the second electrode mixture layer (upper layer) is less than 3.5million which is the range limited by the present disclosure, and inComparative Example 2 in which the molecular weight of CMC contained inthe first electrode mixture layer (lower layer) is 3.5 million exceeding3 million which is the range limited by the present disclosure, it canbe confirmed that the comparative examples are not desirable in terms ofcycle capacity retention due to the increase in resistance.

As in Comparative Example 3, in the case in which 350,000 ultra-lowmolecular weight of CMC is applied to the lower layer, it can be seenthat not only the adhesion of the electrode is greatly reduced, but alsothe capacity retention rate of the battery is greatly reduced. CMChaving a molecular weight of less than 400,000 is mainly used fordispersion and is not suitable for negative electrode slurry productiondue to the low thickening effect thereof. Also, it can be seen that therepulsion effect is significantly reduced by covering the activematerial, resulting in a decrease in the phase stability of the slurryand problems such as SBR aggregation.

As in Comparative Example 4, in the case in which a CMC having a weightaverage molecular weight exceeding 4 million was used in the lowerlayer, it can be seen that the capacity retention rate slightlydecreased due to an increase in resistance.

As set forth above, according to an exemplary embodiment, adhesionbetween the electrode current collector and the active material andcohesion between the active materials may be improved, and theresistance within the electrode may be reduced. As a result, thecapacity and lifespan characteristics of the battery may besignificantly improved.

While this disclosure includes specific examples, it will be apparent toone of ordinary skill in the art that various changes in form anddetails may be made in these examples without departing from the spiritand scope of the claims and their equivalents. The examples describedherein are to be considered in a descriptive sense only, and not forpurposes of limitation. Descriptions of features or aspects in eachexample are to be considered as being applicable to similar features oraspects in other examples. Suitable results may be achieved if thedescribed techniques are performed to have a different order, and/or ifcomponents in a described system, architecture, device, or circuit arecombined in a different manner, and/or replaced or supplemented by othercomponents or their equivalents. Therefore, the scope of the disclosureis defined not by the detailed description, but by the claims and theirequivalents, and all variations within the scope of the claims and theirequivalents are to be construed as being included in the disclosure.

DETAILED DESCRIPTION OF MAIN ELEMENTS

-   -   10: current collector    -   20: first electrode mixture layer    -   30: second electrode mixture

What is claimed is:
 1. An electrode for a secondary battery, comprising:a current collector; a first electrode mixture layer disposed on atleast one surface of the current collector and including a firststyrene-butadiene rubber; and a second electrode mixture layer disposedon the first electrode mixture layer and including a secondstyrene-butadiene rubber, wherein the first styrene-butadiene rubber andthe second styrene-butadiene rubber include a repeating unit of astyrene-derived structure and a repeating unit of a butadiene-derivedstructure, the first styrene-butadiene rubber contains 40 to 90 mol % ofa butadiene monomer based on a total content of a monomer, and thesecond styrene-butadiene rubber has a lower content of a butadienemonomer than the content of the first styrene-butadiene rubber.
 2. Theelectrode for a secondary battery of claim 1, wherein the secondstyrene-butadiene rubber contains 5 to 35 mol % of the butadiene monomerbased on the total amount of the monomer.
 3. The electrode for asecondary battery of claim 1, wherein the content of the secondstyrene-butadiene rubber included in the second electrode mixture layeris the same as or lower than a content of the first styrene-butadienerubber included in the first electrode mixture layer.
 4. The electrodefor a secondary battery of claim 1, wherein the first electrode mixturelayer comprises 2.0 to 5.0% by weight of the first styrene-butadienerubber, based on a total weight of the first electrode mixture layer. 5.The electrode for a secondary battery of claim 1, wherein the secondelectrode mixture layer contains 2.0% by weight or less of the secondstyrene-butadiene rubber, based on a total weight of the secondelectrode mixture layer.
 6. The electrode for a secondary battery ofclaim 1, wherein the first electrode mixture layer comprises 2.0 to 3.5%by weight of the first styrene-butadiene rubber, based on a total weightof the first electrode mixture layer, and the second electrode mixturelayer comprises 1.5% by weight or less of the second styrene-butadienerubber, based on a total weight of the second electrode mixture layer,7. The electrode for a secondary battery of claim 1, wherein at leastone of the first and second styrene-butadiene rubbers includes at leastone of a monomer having a structure derived from acrylate and a monomerhaving a structure derived from acrylonitrile.
 8. The electrode for asecondary battery of claim 1, wherein the thickness of the firstelectrode mixture layer is in the range of 10 to 40 μm.
 9. The electrodefor a secondary battery of claim 1, wherein the thickness of the secondelectrode mixture layer is in the range of 10 to 100 μm, and the firstelectrode mixture layer and the second electrode mixture layer have athickness ratio of 1:1 to
 3. 10. The electrode for a secondary batteryof claim 1, wherein the electrode has an adhesive force of 0.15 to 0.38N/18 mm, between the current collector and a first electrode activematerial layer.
 11. The electrode for a secondary battery of claim 1,wherein the electrode is a negative electrode.
 12. A secondary batterycomprising the electrode of claim
 1. 13. The secondary battery of claim12, wherein the electrode is a negative electrode.